Type annotations in Python

Annotating parameters and returns with modern types.

def mean(values: list[float]) -> float:
    return sum(values) / len(values)


def find(items: dict[str, int], key: str) -> int | None:
    return items.get(key)


def label(value: int, unit: str = "ms") -> str:
    return f"{value}{unit}"


Grid = list[list[int]]


def total(grid: Grid) -> int:
    return sum(sum(row) for row in grid)

How it works

1. list[float] and dict[str, int] type collections.
2. int | None marks an optional return.
3. A type alias like Grid names a reusable shape.

Keywords and builtins used here

• def
• dict
• find
• float
• for
• int
• label
• len
• list
• mean
• return
• str
• sum
• total

The run, in numbers

Lines

17

Characters to type

321

Tokens

114

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 68 seconds.

Type this snippet

Step 1 of 4 in Typing & dataclasses, step 20 of 53 in Pythonic Python.

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